1. Technical Field
The present disclosure relates to an antenna substrate that is used in a multiple-input multiple-output (MIMO) transmission/reception module.
2. Description of the Related Art
In the field of wireless communication, the multiple-input multiple-output (MIMO) method with which transmission and reception are performed using a plurality of transmission antennas and a plurality of reception antennas is known as a method that improves communication speed and reliability. In radar systems also, it is possible to dramatically improve the target detection performance of radar by employing the MIMO method.
In the MIMO method, an antenna having M number of transmission antenna elements and N number of reception antenna elements is treated as a virtual array antenna that is provided with M×N number of antenna elements. However, in this case, the antenna gain of the individual antenna elements is required to be equal. This is because, in a radar system or communication system of the MIMO method, in the case where there are differences in antenna gain among the plurality of antenna elements, the sidelobe signal level increases in received signals and the carrier-to-noise ratio (CN ratio) increases.
Japanese Unexamined Patent Application Publication No. 2014-85317 discloses a technique with which, in a MIMO radar system, the CN ratio of received signals is improved by varying all of the arrangement intervals of a plurality of transmission antenna elements, for example.
In this connection, in a radar system or communication system of the MIMO method in which radio waves of a millimeter wave band are used, the sizes of the antenna elements can be reduced to the wavelength order. Hence, there have been advances in the development of transmission/reception modules in which a semiconductor chip for signal control is mounted on an antenna substrate on which a plurality of antenna elements are arranged corresponding with the MIMO method. In a transmission/reception module such as this, the antenna gain of the individual antenna elements is required to be equal.
An antenna substrate on which a plurality of antenna elements are arranged corresponding with the MIMO method is designed taking into account the number of antenna elements and redundancy. There is a small degree of freedom in the arrangement of the plurality of antenna elements and the arrangement of a plurality of transmission lines connecting each of the antenna elements and each of terminal sections of the semiconductor chip. Therefore, the individual transmission lines are not equal in length. As a result, the antenna gain of the individual antenna elements based upon the terminal sections of the semiconductor chip is not equal, and therefore differences occur in antenna gain among the plurality of antenna elements. Conventionally, in order to eliminate differences in antenna gain, calibrations with respect to signals that are transmitted and received are carried out in signal processing.